1. Field of the Invention
The present invention relates to an apparatus and method for performing xcfx80/4-DQPSK (xcfx80/4-Differential Quadrature Phase Shift Keying) baseband modulation, and particularly to an apparatus and method for building and storing a plurality of parameters and their corresponding tabular data which are simplified previously and using the tabular data to rapidly and correctly modulate input signals.
2. Description of the Related Art
xcfx80/4-DQPSK baseband modulation is widespreadly used in digital wireless baseband modulating technology. For example, USDC and PACS system in North America, and PDC and PHS system in Japan all use xcfx80/4-DQPSK baseband modulation for the design foundation of system wireless modems. The main advantages of xcfx80/4-DQPSK baseband modulation are high utilization of bandwidth, high power utilization and easy to implement. The prior design flow of implementing xcfx80/4-DQPSK baseband modulation is described hereinafter. Firstly, signals Ixe2x80x2(t) and Qxe2x80x2(t) were created through the signal mapping. Secondly, the results are separately sent to two low pass filters to generate in-phase signal I(t) and quadrature-phase signal Q(t). The above-mentioned technology is shown in xe2x80x9cWireless Communications,xe2x80x9d by Theodore S. Rappaport, Prentice Hall, Upper Saddle River, 1996. The prior art method has many disadvantages. For instance, two filters inside the apparatus need a lot of high-speed operations, so that the complexity of the circuit design, accumulation error and power
Accordingly, the object of the invention is to resolve the following drawbacks: (1) the conventional xcfx80/4-DQPSK baseband modulation uses complex mathematical operations to obtain the in-phase signal I(t) and the quadrature-phase signal Q(t), and (2) the parameters of conventional xcfx80/4-DQPSK baseband modulation can not be adjusted as desired. In order to accomplish the object, the present invention proposes an apparatus and method for performing xcfx80/4-DQPSK baseband modulation based on a signal mapping to be simplified. The present invention simplifies the definition and formula of the in-phase signal I(t) and quadrature-phase signal Q(t) in advance, and stores results and corresponding parameters into look-up tables inside a memory. Thus, the present invention only needs a simple logic and control circuit to find out the corresponding data in the memory, and calculates digital in-phase signal In and digital quadrature-phase signal Qn through simple operations. After the process of digital-to-analog conversion, an analog in-phase signal I(t) and analog quadrature-phase signal Q(t) can be obtained. Besides, important parameters of the present invention, such as xcex1 (the roll-off factor of a raised cosine filter, the lager the value is, the faster a degeneration of the signal in time domain, and can use little bits to approach the ideal modulating apparatus), can be adjusted by the user to change the operating characteristic of the system. The present invention only needs small memory capacity, a little operations by the whole circuit, and a little operation time in computation. Therefore, the present invention can transmit data at a high speed. Furthermore, because various results are accurately calculated by the present invention in advance, the accumulation errors will be largely reduced. In conclusion, the present invention reduces the time of operations and memory space, and prevents the accumulation errors occurring easily in the conventional xcfx80/4-DQPSK baseband modulation.
The present invention in baseband modulating apparatus mainly comprises a memory unit and a processing unit. The memory unit stores a plurality of parameters and their corresponding tabular data of a low pass filter, and the parameters and their corresponding tabular data are simplified in advance. The low pass filter is used for reducing the bandwidth of the apparatus and increasing the utilization of the bandwidth. The processing unit bases on input digital data and the tabular data in the memory unit to generate the digital in-phase signal In and the digital quadrature-phase signal Qn.
The baseband modulating method of the present invention mainly comprises: the step of building and storing a plurality of parameters and their corresponding tabular data of a low pass filter, the parameters and their corresponding tabular data being simplified in advance, the low pass filter being used for reducing the bandwidth of the apparatus and increasing the utilization of the bandwidth; the step of sequentially recording the input digital data; the step of according to the input digital data, determining variance of phases after signals are mapped; the step of receiving the tabular data which are associated with the variance of phases; and the step of according to the received tabular data, generating a digital in-phase signal In and a digital quadrature-phase signal Qn.
The present invention also can be implemented by software. Because the present invention has the advantages of simple structure and less operations, the implementation by software also has the advantages.